Opaque films for use in packaging

ABSTRACT

An opaque polymer film is prepared by admixing high crystalline polypropylene with a microvoid causing filler and extruding to form a sheet that is then biaxially stretched to form an opaque film. End uses for these films include soda bottles, candy wrappers and synthetic paper.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. patent application Ser. No.10/949,659, filed Sep. 23, 2004, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to polymer films. The present inventionparticularly relates to polypropylene films useful in packagingapplications.

2. Background of the Art

Polypropylene and polyethylene films are commonly used as packagingmaterials. Applications for such materials include use as labels forplastic containers such as shampoo and soft drink bottles, and plasticbags and wrappings such as those used for candy and other food items.Labels produced from polypropylene or polyethylene films are desirablebecause they are clear and extremely thin films, thus providing a lookthat is similar to silk-screening on the surface at a much lower cost.The polypropylene or polyethylene labels impart a clear, no labelappearance to the plastic bottle. Polypropylene and polyethylene filmsused in these applications typically have a thickness of approximately1.1 to 3.2 mils (27.9 to 81.3 micrometers) and can become easilydeformed. Often, they are used in conjunction with one or more substratematerials.

Polyolefin films can be prepared as opaque films rather than just astransparent films. In some applications, opacity is a desirable propertyfor such packaging films. Opacity can protect materials from beingdegraded by light. For example, packaged foodstuff can be subject todeterioration caused by exposure to light, particularly light having awavelength of up to about 450 nm. Even when a degree of opacity ispresent in the film, spoilage may occur if the film allows passage oftoo much light, therefore highly opaque films are the most desirable forthese purposes.

Another advantage of opaque films is that they can be used as abackground for transparent labels. Opaque films are not always the mostdesirable of surfaces for printing. It is known in the art of preparing,for example, food wrappers to print a label on a transparent film andlaminate or coextrude that film with an opaque film so that the label isdisplayed against an opaque background.

Typically, such opaque polymeric packaging films are multi-layer filmswhich comprise an opaque, thermoplastic polymeric core layer having oneor more skin layers thereon. The skin layers contribute variousdesirable characteristics to the packaging film such as heatsealability, improved appearance, printability or enhanced machinehandling capabilities and the like.

SUMMARY OF THE INVENTION

In one aspect, the present invention is an opaque biaxially orientedpolymer film. The opaque biaxially oriented polymer film is preparedfrom high crystalline polypropylene wherein the opaque biaxiallyoriented polymer film has a thickness of less than 1.25 mil (32micrometers).

In another aspect, the present invention is a label wherein the label ismade using an opaque biaxially oriented polymer film prepared from highcrystalline polypropylene and the opaque biaxially oriented polymer filmhas a thickness of less than 1.25 mil (32 micrometers).

In still another aspect, the present invention is a packaging materialprepared using an opaque biaxially oriented polymer film prepared fromhigh crystalline polypropylene and the opaque biaxially oriented polymerfilm has a thickness of less than 1.25 mil (32 micrometers).

Another aspect of the present invention is an opaque biaxially orientedpolymer film prepared from high crystalline polypropylene wherein theopaque biaxially oriented polymer film has a density of less than 0.55g/cm³.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed understanding and better appreciation of the presentinvention, reference should be made to the following detaileddescription of the invention and the preferred embodiments, taken inconjunction with the accompanying drawings, wherein:

FIG. 1 is a scanning electron microscope (SEM) photomicrograph of across section of an opaque film prepared using a conventionalpolypropylene resin;

FIG. 2 is an SEM photomicrograph of a cross section of an opaque filmprepared using an impact copolymer polypropylene resin;

FIG. 3 is an SEM photomicrograph of a cross section of an opaque film ofthe present invention; and

FIG. 4 is a higher resolution SEM photomicrograph of the same film as inFIG. 3.

DETAILED DESCRIPTION OF INVENTION

High crystalline polypropylene is generally an isotactic configurationof polypropylene, often referenced using the term “i-PP.” The degree ofcrystallinity is an important parameter for establishing relationshipsbetween final product structure and mechanical strength, optical andthermal properties, and also for quality control and productspecification. Solid state NMR remains the predominant method forabsolute measurement of the crystallinity index in polypropylene.Therein, the percentage of isotactic pentads can be determined and thedegree of crystallinity ascertained. As an alternative, the degree ofcrystallinity, average crystallite size and the extent of crystallattice disorder can be determined using wide angle x-ray diffraction.Another way to correlate crystallinity can be determined by measuringthe xylene solubles content of the resin.

The crystalline polypropylenes useful with embodiments of the presentinvention are crystalline polymers comprising propylene as the mainmonomer unit. Included in this group are crystalline propylenehomopolymers with or without including a nucleated agent. Thesepolypropylenes may be obtained using either Ziegler-Natta or metallocenecatalysts.

Regardless of how the degree of crystallinity of the high crystallinepolypropylene is measured, the polymers will have certain properties,the presence of which can be used to differentiate high crystallinepolypropylenes that can be used with embodiments of the presentinvention from those polymers that cannot. For embodiments where thehigh crystalline polypropylenes are obtained using a Ziegler-Nattacatalyst, the high crystalline polypropylene can have a melting point offrom about 155° C. to about 170° C. For the purposes of the presentinvention and for this and any other range of this application,expressly included in this range is any pair of points within the range.For example, the range of about 156° C. to about 169° C. is expresslyincluded in the present invention. The high crystalline polypropylenescan have a heat of fusion of at least 100 joules/g. In anotherembodiment, the high crystalline polypropylenes can have a heat offusion of at least 115 joules/g. In still another embodiment, the highcrystalline polypropylenes can have a heat of fusion of at least 120joules/g. In another embodiment, the high crystalline polypropylenes canhave a heat of fusion of at least 125 joules/g. The high crystallinepolypropylene has a recrystallization temperature of from about 105° C.to about 125° C. These properties are determined using a differentialscanning calorimeter (DSC). For example, the heat of fusion propertiescan be determined using ASTM D-3417-99. The melting point andrecrystallization ranges of the high crystalline polypropylenes usefulwith the present invention can be determined using a DSC method whereina 5 to 10 mg sample is heated and cooled at a rate of 10° C./min. AnyZiegler-Natta derived polypropylene having these properties is a highcrystalline polypropylene for the purposes of the present invention.

For embodiments of the present invention where the high crystallinepolypropylenes are obtained using a metallocene catalyst, the highcrystalline polypropylene will have a melting point of at least about145° C. The metallocene derived high crystalline polypropylenes usefulwith the present invention can have a heat of fusion of at least 70joules/g. The metallocene derived crystalline polypropylene useful withthe present invention has a recrystallization temperature of from about100° C. to about 115° C. These properties are also determined using adifferential scanning calorimeter (DSC). Any metallocene derivedpolypropylene having these properties is a high crystallinepolypropylene for the purposes of the present invention. Exemplary highcrystalline polypropylene useful with the present invention includeATOFINA 3270 polypropylene, ATOFINA EOD01-30 metallocene polypropylene,Amoco 9117, Yuhwa Polypro HF5003 from Korea Petrochemical Ind, Co, Ltd,and also Chisso HF 5010 and Chisso XF 2805 from Chisso Chemical Co.Ltd., Tokyo, Japan. The high crystalline polypropylene, no matterwhether prepared using a metallocene, Ziegler-Natta, or other catalyst,will also have an isotactic index of at least 97 percent.

In embodiments of the present invention, a polymer film is a biaxiallyoriented polymer film. A biaxially oriented film is generally preparedby taking a sheet of polymer and stretching it in two directions.Methods of accomplishing this are well known in the art of preparingpolymer films. For example, U.S. Pat. No. 4,679,283 to Forrest, Jr., etal., which is incorporated herein in its entirety, discloses a devicefor biaxially stretching film. Generally, this process requiresmaintaining the polymer sheet at a temperature wherein the polymer issoft enough to be stretched, but not so soft that the polymer loses allstructural integrity. Generally these devices work by feeding thepolymer sheet through a heater and then accelerating the rate that thepolymer sheet is moved forward while simultaneously stretching the sheetin the direction perpendicular to the direction that the polymer sheetis being moved forward. Any method of biaxially orienting a polymer filmthat is known to those of ordinary skill in the art of preparing polymerfilms to be useful can be used to prepare the films of the presentinvention.

The films of the present invention are opaque. For purposes of thepresent invention, the term opaque film means a film whose lighttransmittance, as tested according to ASTM D1003-00, is less than 50percent due to the presence of microvoids. In another embodiment, thelight transmittance is below 70 percent due to the presence ofmicrovoids. In still another embodiment, the light transmittance of thefilms of the present invention is below 80 percent due to the presenceof microvoids.

The microvoids in the films of the present invention are incorporatedtherein by any method know to be useful to those of ordinary skill inthe art of preparing opaque polymer films. For example, in oneembodiment, a microvoid inducing filler is compounded into the highcrystalline polypropylene polymer directly. In another embodiment, themicrovoid inducing filler is first compounded with a small amount ofpolymer to form a master batch that is then used to compound with highcrystalline polypropylene polymer to form a resin that can be used toprepare the films of the present invention. Any such method can be usedwith the current invention.

Any material that has a higher melting point and is incompatible withhigh crystalline polypropylene film can be used as the microvoidinducing fillers to prepare the films of the present invention. For thepurposes of the present invention, the term incompatible means that thematerial or polymer is in the form of a separate particle or a separatephase in the film. In order to create voids having a volume and geometrythat, after stretching, produce opacity, the fillers have a particlesize of from about 0.5 to about 10 micrometers. In another embodiment,the fillers have a particle size of from about 1.0 to about 7micrometers. In still another embodiment, the fillers have a particlesize of from about 1.5 to about 5 micrometers.

In one embodiment, the microvoid inducing filler is an inorganic solid.The inorganic solid fillers can be selected from aluminum oxide,aluminum sulfate, barium sulfate, calcium carbonate, magnesiumcarbonate, silicates such as aluminum silicate (kaolin clay) andmagnesium silicate (talc), silicon dioxide, titanium dioxide, andmixtures thereof. An embodiment of the present invention includes a filmprepared with calcium carbonate used as a microvoid inducing filler.

The microvoid inducing fillers can be incorporated into the highcrystallinity polypropylene resin using any method known to be useful tothose of ordinary skill in the art of preparing resins for castingpolymer sheets for using making films. For example, the filler can becombined with polypropylene pellets and then reextruded into pellets. Inanother embodiment, the filler can be combined with polypropylene fluffand then further processed into pellets.

The opaque polymer films of the present invention can have severalproperties that can be commercially useful. In one embodiment, theopaque polymer films of the present invention have a lower density thanfilms prepared with other types of polypropylene. For example thepolymer films of the present invention can have a density of from about0.44 g/cm³ to about 0.60 g/cm³. In another embodiment, the polymer filmsof the present invention can have a density of from about 0.48 g/cm³ toabout 0.52 g/cm³. This density is significantly less than that for afilm produced with similar non-crystalline polypropylene resins. Thelower density means that less of the expensive resin can be used toprepare film thus lowering the material costs of the film.

Another excellent property of the films of the present invention istoughness. The opaque polymer films of the present invention can have anapparent toughness of from about 10 to about 14 mPa according.Calculations of tensile toughness are based on the area under thestress-strain curve. This differs from the traditional definition oftensile toughness because traditionally the value being reported istensile toughness at break. The Karo IV tensile toughness values aretensile toughness at test end, regardless if the sample breaks orstretches. This property can be advantageous in applications where anopaque film of the present invention is used as part of a package. Theadditional toughness, as compared to a similar film prepared with, forexample, a conventional rather than a high crystalline polypropylene,could help prevent tearing or punctures of the package.

The opaque films of the present invention retain a surprising amount ofbarrier property notwithstanding the amount of microvoids present in thefilms. While not primarily a vapor barrier material, the opaque polymerfilms of the present invention can functions as such in someapplications. For example, the films of the present invention have awater vapor transfer rate that is nearly the same as that ofconventional polypropylene.

The opaque polymer films of the present invention, can be prepared withadditional pigments to either increase their optical density or changetheir color for aesthetic purposes. Any pigment that can be used withpolypropylene films can be used with the opaque polymer films of thepresent invention. For example, TiO₂ is used to prepare an opaquepolymer film in one embodiment of the present invention. Pigments of anycolor can be used with the present invention.

In addition to pigments, other additives can be used with the presentinvention. Suitable such conventional additives include, by way ofexample, antioxidants, orientation stress modifiers, flame retardants,antistatic agents, antiblock agents, antifoggants, and slip agents.Mixtures of these additives can also be used with the present invention.These additives typically do not contribute to void formation as doesthe calcium carbonate because they have particle sizes that are toosmall.

The polymer films of the present invention can be used alone or incombinations with other materials to form labels and packagingmaterials. For example, in one embodiment, the present invention is alabel including at least two layers wherein the first layer is atransparent layer having thereon printing and the second layer is anopaque polymer film of the present invention. In another embodiment, thepresent invention is a packaging material having at least two layers,one layer of which is an opaque polymer film of the present invention.The films of the present invention are useful in end uses such as sodabottles and food wrappers such as candy wrappers. These films can alsobe used as synthetic paper.

EXAMPLES

The following examples are provided to illustrate the present invention.The examples are not intended to limit the scope of the presentinvention and they should not be so interpreted. Amounts are in weightparts or weight percentages unless otherwise indicated.

Example 1

A 1000 micron cast sheet is prepared by admixing a 7 parts of a highcrystalline polypropylene sold under the trade designation ATOFINA 3270with 3 parts of a master batch consisting of 60 percent CaCO₃; 10percent TiO₂; and 30 percent noncrystalline polypropylene. The resultingresin had a concentration of microvoid forming additive of 20 percent.The cast sheet is formed by feeding the resin through a 1¼ inch (3.2 cm)Welex® extruder using a 10 inch (25.4 cm) die.

The resultant cast sheet is then biaxially oriented to form an opaquefilm using a BRÜCKNER Karo IV laboratory orienter. The sheet isstretched 5×8 at 150° C. The opaque film is then tested for certainphysical properties, and the results of the testing are displayed belowin the table.

Comparative Example I

An opaque film is prepared substantially identically to Example 1 exceptthat instead of a high crystalline polypropylene, a conventionalpolypropylene sold under the trade designation ATOFINA 3371 is used andthe film is oriented at 145° C.

Comparative Example II

An opaque film is prepared substantially identically to Example 1 exceptthat instead of a high crystalline polypropylene, an impactpolypropylene copolymer sold under the trade designation ATOFINA 4320 isused and the film is oriented at 145° C.

Comments Regarding the Examples

As can be seen from the examples above, the films of the presentinvention are significantly more opaque and have a greater apparenttoughness at the same loadings of the microvoid forming additive. Thephotomicrograph of the cross sections of the films and comparativeexamples shows the greater depth and breadth of the microvoids of thefilms of the present invention. An opaque film of the present inventioncan have substantially more microvoids than either conventionalpolypropylene or impact polypropylene copolymer as is shown in FIGS.1-4.

TABLE Example Comparative Comparative Property Units 1 Example I ExampleII Density g/cm³ 0.51 0.64 0.61 Thickness micron 45 38 40 LightTransmittance¹ percent 19 32 27 Whiteness² percent 91 87 88 Gloss @45°³percent 44 21 17 1% Sec Modulus MPa 1000 950 590 MD⁴ 1% Sec Modulus MPa1900 1900 1300 TD⁵ Tensile at Break MPa 60 65 55 MD⁶ Tensile at BreakTD⁷ MPa 120 130 110 Elongation at Break percent 70 90 70 MD⁸ Elongationat Break percent 15 20 20 TD⁹ Water Vapor g · 25.4μ/(m² · day) 9.0 8.318.7 Transmission Rate¹⁰ Oxygen cc · 25.4μ/(m² · day) 4200 2400 9020Transmission Rate¹¹ Apparent MPa 12.0 10.1 7.5 Toughness¹² ¹ASTMD1003-00 ²ASTM E313-00 ³ASTM D2457-03 ⁴ASTM D882-02 ⁵ASTM D882-02 ⁶ASTMD882-02 ⁷ASTM D882-02 ⁸ASTM D882-02 ⁹ASTM D882-02 ¹⁰ASTM F1249-01 ¹¹ASTMD3985-02e1 ¹²Calculations of apparent toughness are based on the areaunder the stress-strain curve.

1. An opaque biaxially oriented polymer film comprising a highcrystalline polypropylene having a heat of fusion of at least 70joules/g and prepared using a metallocene catalyst, wherein the film hasa thickness of less than 1.25 mil and wherein the film has a density offrom about 0.44 to about 0.55 g/cm³.
 2. The opaque biaxially orientedpolymer film of claim 1, wherein the high crystalline polypropyleneexhibits a melting point of from about 155° C. to about 170° C.
 3. Theopaque biaxially oriented polymer film of claim 1, wherein the opaquebiaxially oriented polymer film exhibits a light transmittance, measuredaccording to ASTM D1003-00, of less than 80 percent.
 4. The opaquebiaxially oriented polymer film of claim 1, wherein the high crystallinepolypropylene exhibits a recrystallization temperature of from about105° C. to about 125° C.
 5. The opaque biaxially oriented polymer filmof claim 1, wherein the polymer film further comprises a pigment.
 6. Theopaque biaxially oriented polymer film of claim 5, wherein the pigmentis TiO₂.
 7. A label formed from the opaque biaxially oriented polymerfilm of claim 1.